Method and apparatus for forming complex contour structural assemblies

ABSTRACT

A method and associated preform for forming a structural assembly that defines a complex contour are provided. The preform and, hence, the structural assembly includes first and second skin members and a cellular core member between the first and second skin members. The skin members and/or the core member can be formed to a shape that corresponds to the desired contour of the structural member before the preform is assembled, e.g., by superplastic forming. That is, the first skin member can be disposed against a contour surface of a die to restrain the preform to the contour defined by the surface. A pressurized fluid is provided against the second skin member to urge the preform against the contour surface. A pressurized fluid is also injected in the first chamber to support the cellular core member. The preform is heated to a bonding temperature before or after being disposed against the contour surface so that the core and skin members are bonded to form the structural assembly.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to structural assemblies and, moreparticularly, relates to a method and preform for forming a complexcontour structural assembly.

2. Description of Related Art

Honeycomb panels are used in a variety of applications, especially wherestrength and minimum weight are required. A conventional honeycomb paneltypically includes a core between first and second skins or face sheets.The core is a cellular member, i.e., the core defines a number ofinternal cells. In some cases, each cell is six-sided and extends in alongitudinal direction parallel to the other cells of the core. However,other cores can also be formed and used. Such panels can be formed of avariety of materials, including aluminum, titanium, and the like.

Metallic honeycomb panels are typically formed in a flat or gentlycontoured configuration. For example, in some cases, the core is formedfrom a flat sheet and bonded to the face sheets during a combinedforming and bonding operation. In particular, the core and face sheetscan be superplastically formed and diffusion bonded or brazed to form aflat or gently contoured honeycomb panel. Such forming and bondingoperations are described in U.S. Pat. No. 4,117,970, entitled “Methodfor Fabrication of Honeycomb Structures,” which issued on Oct. 3, 1978;U.S. Pat. No. 5,420,400, entitled “Combined Inductive Heating Cycle forSequential Forming the Brazing,” which issued on May 30, 1995; U.S. Pat.No. 5,700,995, entitled “Superplastically Formed Part,” which issued onDec. 23, 1997; U.S. Pat. No. 5,705,794, entitled “Combined HeatingCycles to Improve Efficiency in Inductive Heating Operations,” whichissued on Jan. 6, 1998; U.S. Pat. No. 5,914,064, entitled “CombinedCycle for Forming and Annealing” which issued on Jun. 22, 1999; and U.S.Pat. No. 6,337,471, entitled “Combined Superplastic Forming and AdhesiveBonding” which issued on Jan. 8, 2002.

In some cases, the honeycomb panel can then be formed to a contouredshape, i.e., so that each of the face sheets define a two- orthree-dimensional curve. The degree of contour that can be imparted tosuch a structural panel is dependent on such factors as the size andconfiguration of the panel, the material properties of the panel, thetype of forming process, and the like. If excessively formed,undesirable changes in the configuration of the panel may result, suchas crushing of the core, buckling of the face sheets, or destruction ofthe joints between the core and face sheets.

The structural assemblies for some applications define complex contoursthat are difficult or impossible to form by conventional manufacturingmethods. For example, U.S. Pat. No. 6,371,411, issued Apr. 16, 2002,U.S. Pat. No. 6,457,676, issued Oct. 1, 2002, and U.S. Pat. No.6,688,558, issued Feb. 10, 2004, each titled “Method and apparatus foraircraft inlet ice protection,” describe an ice protection system for aninlet for an aircraft engine, which inlet can be partially lined with ahoneycomb core sandwiched between sheets, thereby improving the acousticattenuation of the inlet. In some cases, the core is provided inmultiple portions, in part, to facilitate manufacture.

Thus, there exists a need for an improved method and preform forproducing metallic complex contour structural assemblies, includingcontours not typically formable by conventional methods. The methodshould be capable of producing structural assemblies with cellular coresand should be compatible with superplastically formable materials suchas titanium.

SUMMARY OF THE INVENTION

The present invention provides a method and preform for forming astructural assembly that defines a complex contour. The preform and,hence, the structural assembly includes first and second skin membersand a cellular core member between the skin members. The skin membersand/or the core member can be formed to a shape that corresponds to thedesired contour of the structural member before the preform isassembled, e.g., by superplastic forming. Thus, complex contours thatwould be difficult or impossible to form from a flat preform can beproduced.

According to one embodiment of the present invention, the core member isdisposed between the skin members, and a periphery of the skin membersis connected to define a first gas chamber between the skin members. Acloseout member can be connected to the second skin member opposite thefirst skin member to define a second chamber. The first skin member isthen disposed against a contour surface of a die, thereby restrainingthe preform to the contour of the surface, and a pressurized fluid isprovided against the second skin member to urge the preform against thecontour surface, e.g., by injecting a pressurized fluid in the secondchamber. A pressurized fluid is also injected in the first chamber tosupport the cellular core member. The preform is heated to a bondingtemperature before or after being disposed against the contour surfaceso that the core and skin members are bonded to form the structuralassembly.

The pressure provided against the second skin member for urging thepreform against the contour surface can be greater than the pressureprovided in the first chamber. For example, a pressure of at least about10 psi can be provided against the second skin member, and the pressurein the first chamber can be about 0 and 1 psi less than the pressureprovided against the second skin member. The preform can be heated to adiffusion bonding temperature of the preform to thereby diffusion bondthe core member to the skin members. Alternatively, a braze material canbe provided between the core member and the skin members, and thepreform can be heated to a brazing temperature of the braze material sothat the core member is brazed to the skin members.

According to one embodiment of the present invention, the closeoutmember is formed of a cellular core member between contour skin members.Thus, the closeout member can be restrained against a second contour diesurface while the core member of the closeout member is bonded to theskin members of the closeout member, e.g., so that the closeout membercan be used to form a structural assembly having desired dimensions.After bonding, the preform can be trimmed to form one or more structuralassemblies. The dies surface(s) for restraining the preform can bedefined by first and second dies, which can be closed to form a diecavity. In some cases, the die cavity can be substantially sealed and apartial vacuum can be formed in the cavity.

The present invention according to another embodiment provides a preformfor forming a structural assembly defining a complex contour. Thepreform includes first and second contour skin members, each defining acontour corresponding to the structural assembly. The skin members areconnected to define a first gas chamber, with a core member disposed inthe gas chamber. A closeout member is connected to the second skinmember and defines a second gas chamber. A first gas connection in fluidcommunication with the first chamber is configured to deliver apressurized fluid to the first chamber for supporting the core member. Asecond gas connection in fluid communication with the second chamber isconfigured to deliver a pressurized fluid to the second chamber to urgethe preform against a contour surface of a die. The skin members can besuperplastically formed, and the skin members and/or the core member canbe formed of similar or dissimilar materials, including titanium. Thecore member can be bonded to the skin members by diffusion bonds orbraze joints.

In one embodiment, the closeout member includes third and fourth contourskin members that are connected to define a third gas chamber with acore member disposed in the third gas chamber. The third and fourth skinmembers are joined to the first and second skin members to form thesecond gas chamber between the first and third skin members. Thus, thefirst and second skin members can be separated from the third and fourthskin members to form two structural assemblies.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other advantages and features of the invention, andthe manner in which the same are accomplished, will become more readilyapparent upon consideration of the following detail description of theinvention taken in conjunction with the accompanying drawings, whichillustrate preferred and exemplary embodiments and which are notnecessarily drawn to scale, wherein:

FIG. 1 is a section view illustrating a system including a die set formanufacturing a structural assembly using a preform according to oneembodiment of the present invention;

FIG. 1A is a plan view illustrating the system of FIG. 1;

FIG. 2 is a perspective view illustrating the preform of FIG. 1;

FIG. 2A is another perspective view illustrating the preform of FIG. 1;

FIG. 3 is a chart indicating the variation in pressure provided in thepreform during a bonding process according to one embodiment of thepresent invention;

FIG. 4 is a perspective view illustrating a structural assemblyaccording to one embodiment of the present invention;

FIG. 5 is a perspective view illustrating an inlet lip for an aircraftengine formed of four of the structural assemblies of FIG. 4; and

FIG. 6 is a section view illustrating a die for manufacturing twostructural assemblies from a preform according to another embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention now will be described more fully with reference tothe accompanying drawings, in which some, but not all embodiments of theinvention are shown. This invention may be embodied in many differentforms and should not be construed as limited to the embodiments setforth. Like numbers refer to like elements throughout.

Referring now to the drawings, and in particular to FIG. 1, there isshown a system 10 according to one embodiment of the present invention,which can be used to form a structural assembly from a preform. Inparticular, the preform 40 illustrated in FIG. 1 can be used tomanufacture the monolithic structural assembly 70 that is illustrated inFIG. 4, which can be a lip or a portion of a lip for a nacelle. Thestructural assemblies according to the present invention can definevarious contours and configurations. In particular, the structuralassemblies can define complex contours, such as contours that includeone or more two- or three-dimensional curves. The structural assembliescan be used in a variety of industries and applications including, butnot limited, in connection with the manufacture of aircraft and otheraerospace structures and vehicles. Further, the structural assembliescan be used individually or in combination with other structures anddevices.

The system 10 illustrated in FIG. 1 includes a die set 12 having firstand second dies 14, 16, which cooperatively define a die cavity 18. Thedie set 12 is configured to be adjusted between open and closedpositions so that the die cavity 18 can be opened to receive the preform40 and then closed during the forming operation. For example, the seconddie 16 can be lifted from the first die 14, or the first die 14 can belowered relative to the second die 16. In other embodiments of thepresent invention, the dies 14, 16 can be configured in a horizontalconfiguration such that one or both of the dies 14, 16 can be movedhorizontally to open the die cavity 18. A support structure 20 such as ahydraulic press can be provided for clamping the die set 12 in theclosed position, e.g., by pressing the dies 14, 16 together or otherwisesecuring the dies 14, 16 in the closed position. The hydraulic press orother support structure 20 should be capable of maintaining the positionof the dies 14, 16 while internal pressures are achieved in the diecavity 18.

The dies 14, 16 can be formed of a variety of materials including, e.g.,ceramic, metals, and the like. For example, in the embodimentillustrated in FIG. 1, the first die 14 is formed of a cast ceramic witha low thermal expansion and a high thermal insulation. The second die 16is formed of a high temperature alloy of stainless steel. The additionalsupport structure 20 can also be provided to maintain the shape of thedies 14, 16 and prevent damage to the dies during operation andhandling, such as is described in U.S. Pat. No. 5,683,608, entitled“Ceramic Die for Induction Heating Work Cells,” which issued on Nov. 4,1997, and U.S. Pat. No. 6,528,771, entitled “System and Method forControlling an Induction Heating Process,” which issued Mar. 4, 2003,both of which are assigned to the assignee of the present invention, andthe entirety of each of which is incorporated by reference. While thedies 14, 16 are described to be formed of different materials in theforegoing examples, the dies 14, 16 may be formed of the same materialin other embodiments.

The die set 12 can be heated before or after the preform 40 is disposedin the die cavity 18. For example, the die set 12 can be disposed in anoven or other heating device before or after the preform 40 is loadedinto the die cavity 18. Alternatively, the system 10 can include aheater, such as a heater that is integral to the die set 12, asdescribed in assignee's copending patent application titled “System andMethod for Processing a Preform in Vacuum Vessel to Produce a StructuralAssembly,” U.S. patent application Ser. No. ______, which is filedconcurrently and the entirety of which is incorporated by reference. Insome cases, the die set 12 can be disposed in a closed vessel that isconfigured to be at least partially evacuated to provide a partialvacuum for forming. Such a vacuum vessel is also described in “Systemand Method for Processing a Preform in Vacuum Vessel to Produce aStructural Assembly,” filed concurrently.

As illustrated in FIG. 1, the preform 40 includes first and second skinmembers 42, 44, or face sheets, with a cellular core member 46 betweenthe skin members. The first and second skin members 42, 44 can definevarious contours and configurations according to the intendedapplication of the finished structural assembly 70. In particular, eachof the skin members 42, 44 can define a complex contour, which caninclude one or more two- or three-dimensional curves or angles. In somecases, the skin members 42, 44 can be formed by superplastic formingbefore assembly of the preform 40. That is, each skin member 42, 44 canbe heated to its superplastic forming temperature and formed to adesired shape prior to assembly of the preform 40.

Superplastic forming (“SPF”) generally refers to a process for formingplastics and metals, including titanium, aluminum, and alloys of suchmetals, that exhibit superplastic behavior at certain temperatures,i.e., large elongations (of 2000% or more). The SPF process can be usedfor forming a single SPF sheet or an SPF pack that includes multiplelayered sheets. During the SPF process, the SPF sheet or pack is placedinto a shaping die and heated to a sufficiently high temperature withinthe superplasticity range of the material to soften the material.Pressurized heated gas is then injected against the material, andpossibly into the pack, if applicable, thereby causing the sheet or packto be urged against the die. In some cases, the portions of the sheetsbrought into contact during the forming are bonded or brazed togetherunder the forming or post-forming conditions. The formed sheet or packis then cooled and removed from the die and final machining steps areperformed, such as edge trimming. Advantageously, the SPF process can beused to form structures that can satisfy narrow shape and tolerancerequirements without substantial additional machining. Such SPF andcombined SPF-bonding cycles are described in U.S. Pat. No. 4,117,970,entitled “Method for Fabrication of Honeycomb Structures,” which issuedon Oct. 3, 1978; U.S. Pat. No. 5,410,132, entitled “Superplastic FormingUsing Induction Heating,” which issued on Apr. 25, 1995; U.S. Pat. No.5,420,400, entitled “Combined Inductive Heating Cycle for SequentialForming the Brazing,” which issued on May 30, 1995; U.S. Pat. No.5,700,995, entitled “Superplastically Formed Part,” which issued on Dec.23, 1997; U.S. Pat. No. 5,705,794, entitled “Combined Heating Cycles toImprove Efficiency in Inductive Heating Operations,” which issued onJan. 6, 1998; U.S. Pat. No. 5,914,064, entitled “Combined Cycle forForming and Annealing” which issued on Jun. 22, 1999; and U.S. Pat. No.6,337,471, entitled “Combined Superplastic Forming and Adhesive Bonding”which issued on Jan. 8, 2002, each of which is assigned to the assigneeof the present invention, and the entirety of each of which isincorporated by reference.

Alternatively, the skin members 42, 44 can be formed by otherconventional forming processes. However, superplastic forming canproduce shapes that are difficult or impossible to form using some otherforming techniques. In any case, the skin members 42, 44 cansubstantially define the desired contours of the finished structuralassembly 70 so that no forming, or minimal forming, of the skin members42, 44 is required after the preform 40 is assembled. That is, the shapeof the preform 40 can substantially correspond to the finished shape ofthe structural assembly 70 so that the shape of the preform 40 must onlybe maintained during the subsequent bonding operation. In particular,the skin members 42, 44 can define deeply contoured curves, such as isillustrated in FIG. 1, which may be difficult or impossible to formafter assembly of the preform 40. For example, the first skin member 42can define a convex contour that corresponds to a first surface of thefinished structural assembly 70, and the second skin member 44 candefine a concave contour that corresponds to an opposite, second surfaceof the structural assembly 70.

During assembly of the preform 40, the skin members 42, 44 are disposedwith the cellular core member 46 between the skin members. That is, theskin members 42, 44 are disposed on opposite sides of the core member 46so that the core member 46 is disposed in a first chamber 48 definedbetween the skin members 42, 44. The cellular core 46 member typicallyhas a honeycomb configuration that defines a plurality of parallelsix-sided cells 50, each extending in a longitudinal directionsubstantially parallel to the adjacent cells 50, though other cellularconfigurations can alternatively be formed. The core member 46 can be aflexible structure, i.e., so that the core member 46 can be bent to theconfiguration of the skin members 42, 44 without crushing the cells 50.Thus, the core member 46 can be formed at least approximately to theconfiguration of the preform 40 before assembly of the preform 40, e.g.,by a roll forming process. Alternatively, the core member 46 can beconstructed in the configuration of the preform 40 so that no subsequentforming is required before assembly of the preform 40.

The preform 40 also includes a closeout member 52, i.e., a memberdisposed partially against the second skin member 44 opposite from thefirst skin member 42. The closeout member 52 can be a removable memberthat is provided during formation of the assembly 70 and then removed.As shown in FIG. 1, the closeout member 52 can be a flat sheet, suchthat the closeout member 52 and the second skin member 44 define asecond chamber 54. In other embodiments of the present invention, thecloseout member 52 can have other configurations, such as the shape ofan additional structural assembly, as described below in connection withFIG. 6.

The skin members 42, 44, cellular core member 46, and the closeoutmember 52 can be formed of various materials including, but not limitedto, aluminum, titanium, alloys that include aluminum or titanium, andthe like. Further, the members 42, 44, 46, 52 can be formed of similaror dissimilar materials. For example, according to one embodiment of thepresent invention, the skin members 42, 44 can each be formed ofTi-6A1-4V, and the core member 46 can be formed of Ti-3A1-2.5V. If thecloseout member 52 is to be removed from the finished assembly 70 anddiscarded or recycled after bonding by the system 10, the closeoutmember 52 can be formed of a variety of cost-efficient materials thatcan be sufficiently bonded to the rest of the preform 40. The particularmaterials to be used for each member 42, 44, 46, 52 can be selected tofacilitate the manufacture of the assembly 70 and to provide in thefinished assembly 70 the desired material properties and characteristicsincluding strength, corrosion resistance, and the like.

The members 42, 44, 46, 52 are connected in the desired configuration toform the preform 40. For example, with the cellular core member 46disposed between the first and second skin members 42, 44, the skinmembers 42, 44 and the closeout member 52 can be clamped in place andthen connected at a periphery 56 by a weld joint 58 as shown in FIGS. 2and 2A. That is, a substantially continuous weld joint 58 can be formedalong the transverse edges 60 and the longitudinal edges 62 of themembers 42, 44, 52. The weld joint 58 can be formed by various weldingprocesses, including resistance seam welding, friction stir welding orother types of friction welding, fusion welding, gas arc welding, andthe like. In any case, one or more gas connections 64, 66 can be formedthrough the weld joint 58, for example, by providing tubes, pipes, orother ports extending to the internal chambers 48, 54 of the preform 40so that the chambers 48, 54 between the members 42, 44, 52 are otherwisesubstantially sealed gas chambers. In particular, first gas connections64 can be provided in fluid communication with the first gas chamber 48,and second gas connections 66 can be provided in fluid communicationwith the second gas chamber 54. The tubes that provide the gasconnections can be fusion welded or otherwise joined to the preform 40.

Referring again to FIG. 1, the preform 40 is placed in the die cavity 18with the first skin member 42 disposed against the first die 14 and thecloseout member 52 disposed against the second die 16. In some cases, aparting agent such as boron nitride can be coated on the outer surfacesof the members 42, 52 and/or the inner surfaces of the dies 14, 16. Thefirst die 14 typically defines a contour surface 15 that corresponds tothe desired shape of the structural assembly 70 and, hence, the contourof the first skin member 42. The first and second gas chambers 48, 54are fluidly connected to one or more sources 80, 82 for pressurizedfluid, as shown in FIG. 1A, typically using stainless steel tubing. Byproviding pressurized fluid to the chambers 48, 54 through the gasconnections 64, 66, the various members of the preform 40 can be urgedtogether to form bonds. Multiple gas connections 64, 66 can be providedfor each chamber 48, 54, e.g., so that fluid can be supplied through oneof the gas connections 64, 66, and the pressure in each chamber 48, 54can be sensed or detected through another one of the connections 64, 66.

According to one method of the present invention, the preform 40 isassembled, such that the skin members 42, 44 and closeout member 52 areconnected and the gas chambers 48, 54 are substantially sealed exceptfor the gas connections 64, 66. The preform 40 is placed in the diecavity 18, and the die set 12 is adjusted to a closed position as shownin FIG. 1 so that the first skin member 42 is disposed against thecontour surface 15 of the first die 14, and the closeout member 52 isdisposed against the second die 16. The dies 14, 16 are secured in thisposition, e.g., by urging the two dies 14, 16 together with a hydraulicram or otherwise securing the dies 14, 16. The die set 12 and/or thepreform 40 can be heated before or after the preform 40 is disposed inthe die set 12. In particular, the preform 40 can be heated to atemperature at which diffusion bonding or brazing of at least some ofthe members 42, 44, 46, 52 can be performed. For example, the die set 12can be heated in a furnace before or after the preform 40 is placed inthe die set. Alternatively, the preform 40 can be heated using a heaterthat is integral to the die set 12, or the preform 40 can be heated byproviding an electromagnetic field that induces an electric current inthe preform 40 or in a separate susceptor member disposed in thermalcommunication with the preform 40. The use of such susceptors forheating preforms in a die is further described in U.S. Pat. Nos.5,705,794; 5,914,064; and 6,337,471, noted above.

The temperature to which the preform 40 is heated can be determinedaccording to the type of material of the preform 40, the type of bondingto be performed, and the like. For example, titanium typically can besuperplastically formed and diffusion bonded at a temperature of betweenabout 1600° and 1700° F. For other materials and other forming andbonding operations, a higher or lower temperature can be provided.

Pressurized fluid is also delivered to the gas chambers 48, 54 of thepreform 40 to achieve the required contact between the members 42, 44,46, 52 of the preform 40 for bonding. For example, as shown in FIG. 1A,first and second pressurized gas sources 80, 82 can be connected to thefirst and second gas chambers 48, 54, respectively, via pressureregulation devices 84, 86 that are configured to separately control thepressure in each chamber 48, 54. While separate gas sources 80, 82 areillustrated, a single source can instead be configured to pressurizeboth of the chambers 48, 54 via one or more pressure regulation devices.In some cases, the gas in the die cavity 18 in and around the gaschambers 48, 54 of the preform 40 can be purged by repeatedly vacuuminggas from the die cavity 18 and refilling the die cavity 18 (includingthe chambers 48, 54) with an inert gas such as argon. Then, the secondgas source 82 can deliver a pressure to the second gas chamber 54,thereby expanding the chamber 54 and urging the closeout member 52 andsecond skin member 44 outward. The second skin member 44 is urged towardthe first skin member 42 and, thus, against the core member 46, and thefirst skin member 42 is urged against the contour surface 15 of thefirst die 14. Thus, the preform 40 is held against the contour surface15 and, hence, maintained in the desired configuration of the structuralassembly 70 during the bonding operation. In fact, if the preform 40does not conform to the contour of the surface 15 when placed in the diecavity 18, the preform 40 can be formed against the contour surface 15and thereby formed to the desired configuration of the structuralassembly 70. Further, by virtue of the pressure in the second chamber54, the core member 46 is compressed between the first and second skinmembers 42, 44 and can thus be joined by diffusion bonding or brazing.

The first pressurized gas source 80 can deliver a pressure to the firstgas chamber, e.g., to resist crushing or other damage to the core member46. Generally, the pressure in the first gas chamber 48 is lower thanthe pressure in the second gas chamber 54 so that the second skin member44 is pressed against the core member 46. That is, a pressuredifferential between the first and second gas chambers 46, 54 is used tocontrol the amount of compressive force applied to the core member 46.This technique enhances the ability to achieve intimate contact betweenthe skin and core members 42, 44, 46, even if each of the members 42,44, 46 defines a complex shape, while maintaining the form of thepreform 40 by compressing it against the contour surface 15 of the die14.

For example, a controller 90, such as a computer, programmable logicdevice, or other processor, can be provided for controlling the bondingoperation. In particular, the controller 90 can be configured tocommunicate electrically with the gas source(s) 80, 82 and/or theregulation device(s) 84, 86 to control the pressure in the two chambers48, 54 of the preform 40. For example, the controller 90 can increasethe pressure in the second gas chamber 54 while maintaining no more thana maximum differential relative to the pressure in the first gas chamber48, such as a differential of about 0.40 psi or 0.60 psi greater thanthe pressure in the first gas chamber 48.

As a result, the preform 40 is sufficiently urged against the contoursurface 15 of the die 14 for restraining the preform 40 to the desiredshape. Further, the skin members 42, 44 are put in sufficient contactwith the core member 46 to achieve bonding without the core member 46being crushed between the skin members 42, 44.

As noted above, the skin members 42, 44 can be joined to the core member46 by diffusion bonding or brazing. Diffusion bonding generally refersto a bonding operation in which the members to be bonded are heated to atemperature less than the melting temperature of each member and pressedin intimate contact to form a bond between the members. Brazinggenerally refers to a bonding operation in which a braze material isprovided between the members that are to be joined, and the members andbraze material are heated to a temperature higher than the meltingtemperature of the braze material but lower than the melting temperatureof the members being joined. Thus, a diffusion bond can be formedbetween the core member 46 and the skin members 42, 44 of the preform 40by heating the members 42, 44, 46 and urging them together withsufficient pressure in the gas chambers 48, 54. Brazing can be performedsimilarly, but generally requires that an additional braze material 68be provided between the members 42, 44, 46, e.g., at the interface ofthe core member 46 and the skin members 42, 44 as indicated in FIG. 1.The braze material 68 can be selectively provided where joints are to beformed, or the braze material 68 can be provided nonselectively betweenthe members 42, 44, 46 to be joined. For example, the braze material 68can be provided as a sheet of material between the members 42, 44, 46,as a powder disposed between the members 42, 44, 46, by plating thebraze material 68 on the surface of one or more of the members 42, 44,46, or otherwise.

The variations in pressure provided in the gas chambers 48, 54 of thepreform 40 during one exemplary bonding process are illustrated in FIG.3. In the “Load” operation, the preform 40 is loaded into the die set 12as described above. The initial pressure (P2) in the second chamber 54between the second skin member 44 and the closeout member 52 is set toabout 0 psi, i.e., atmospheric pressure, and the initial pressure (P1)in the first chamber 48 between the skin members 42, 44 is set to about−0.20 psi. During each subsequent operation, the pressures P1, P2 areadjusted to new values during a “Ramp” time and held at the newpressures for a “Soak” time. For example, during operation #1, thepressure P2 in the second chamber 54 is increased to 14 psi during aramp period of 15 minutes. Then, the pressure P2 is maintained for 263minutes, until the start of operation #4, thereby pressing the skinmembers 42, 44 against the core member 46 and pressing the first skinmember 42 against the contour surface 15 of the first die 14. Meanwhile,during operation #1, the first pressure P1 in the first chamber 48 isincreased to 13.40 psi during the first 15-minute ramp period. Duringoperation #2, the first pressure P1 is increased to 13.60 psi during a 1minute ramp period and then held for 120 minutes. During operation #3,the first pressure P1 is further increased to 13.82 psi and held at thatpressure for 143 minutes. In operation #4, the first pressure P1 isreduced back to −0.20 psi during a ramp period of 15 minutes.

It can be seen that the difference between the pressures P1, P2 in thetwo gas chambers 48, 54 can be limited, even while the pressures ineither or both of the chambers 48, 54 fluctuate. For example, in theillustrated embodiment of FIG. 3, the second pressure P2 is higher thanthe first pressure P1 throughout the bonding process; however, thesecond pressure P2 does not exceed the first pressure P1 by more than0.60 psi at any time during the process. The particular difference inthe pressures P1, P2 can be achieved to sufficiently bond the skinmembers 42, 44 to the core member 46 while avoiding crushing of thecells 50 of the core member 46. The overall pressures P1, P2 can beselected to provide a sufficient force between the preform 40 and thefirst die 14 so that the preform 40 is constrained to the contour of thesurface 15 during bonding. In addition, one or both of the chambers 48,54 can be evacuated during forming to provide the necessary forces onthe preform 40. For example, the first chamber 48 can be partiallyevacuated to form a partial vacuum in the chamber 48, thereby providinga pressure differential with the second chamber 54. In some cases, apressure in the second chamber 54 of about atmospheric pressure may besufficient.

After the bonding operation is complete, the preform 40 can be removedfrom the die cavity 18, typically after the preform 40 is at leastpartially cooled in the die set 12 to prevent distortion of the preform40 during or after removal. Depending on the material of the preform 40,it may be possible to remove the preform 40 with little cooling in thedie set 12. Regardless of whether the preform 40 is cooled in or out ofthe die cavity 18, the rate of cooling of the preform 40 can becontrolled. For example, the system 10 can include a device for coolingthe dies 14, 16 and, hence, the preform 40, such as a pump forcirculating a coolant fluid through passages defined by the dies 14, 16.Such a cooling operation is described, e.g., in U.S. Pat. No. 6,528,771,noted above. If the preform 40 is removed from the die set 12 while hot,the preform 40 can be wrapped in blankets or otherwise insulated tolimit the rate of cooling. Alternatively, the rate of convective coolingof the preform 40 can be enhanced by inducing air circulation proximatethe preform 40.

The preform 40 can also be machined or otherwise trimmed to the desiredconfiguration of the structural assembly 70. In particular, the closeoutmember 52 can be trimmed from the preform 40 and/or the weldedperipheral portions 56 of the members 42, 44 can be removed from thepreform 40. For example, the preform 40 of FIGS. 1, 2, and 2A can bemachined to form the monolithic structural assembly 70 shown in FIG. 4.As illustrated in FIG. 4, the structural assembly 70 defines athree-dimensionally curved contour, i.e., a contour curved about atleast two non-parallel axes. In particular, the structural assembly 70illustrated in FIG. 4 has a cross section that defines a generallyC-shaped curve having a radial center 45, and the center 45 defines acenterline that extends along an arc of 90°. In other embodiments, thecenter 45 can extend through an arc that is lesser or greater, includinga 360° arc. Further, the arc defined by the centerline of the assemblycan be circular or non-circular.

In some cases, the structural assembly 70 can be further assembled withother similar structural assemblies. For example, FIG. 5 illustrates aninlet lip 72 for a nacelle of an aircraft engine formed of four of thestructural assemblies 70 of FIG. 4. That is, the four structuralassemblies 70 can be welded or otherwise joined to form the inlet lip 72such that the inlet lip 72 has a continuous convex outer mold linesurface 74, defined by the first skin members 42 of the structuralassemblies 70, and a continuous concave inner mold line surface 76,defined by the second skin members 44 of the structural assemblies 70.The cellular core members 46, which are disposed between the outer andinner mold line surfaces 74, 76, can provide increased acousticattenuation and/or can be used to transmit and direct hot gas to reduceice formation on the inlet, as described in U.S. Pat. No. 6,371,411,issued Apr. 16, 2002; U.S. Pat. No. 6,457,676, issued Oct. 1, 2002; andU.S. Pat. No. 6,688,558, issued Feb. 10, 2004, each of which is titled“Method and apparatus for aircraft inlet ice protection,” each of whichis assigned to the assignee of the present invention, and the entiretyof each of which is incorporated by reference.

The closeout member 52 of the preform 40 illustrated in FIGS. 2 and 2Ais removed after bonding to produce the structural assembly 70 of FIG.4. According to other embodiments of the present invention, the closeoutmember 52 can instead be bonded, formed, or otherwise processed duringtreatment of the preform 40 and used to produce a structural assembly.In this regard, the preform 40 illustrated in FIG. 6 includes a closeoutmember that includes two skin members 42 a, 44 a and a core member 46 aconfigured similar to the skin and core members 42, 44, 46. In fact, thethird and fourth skin members 42 a, 44 a and core member 46 a of thecloseout member 52 can have a configuration that is similar ordissimilar to the first and second skin members 42, 44 and core member46. Further, the second die 16 can define a contour surface 17 thatcorresponds to the shape of the fourth skin member 44 a and defines adesired contour of the structural assembly 70. In this way, the preform40 can define three gas chambers, i.e., a third gas chamber 48 a betweenthe third and fourth skin members 42 a, 44 a, in addition to the firstand second gas chambers 48, 54. A gas connection to the third gaschamber 48 a can be provided so that the third chamber 48 a can bepressurized independently of the first and second chambers 48, 54.During processing, the third gas chamber 48 a can be pressurized to apressure similar to the pressure in the first chamber 48. That is, thefirst and third chambers 48, 48 a can be pressurized to prevent crushingof the core members 46, 46 a using sources 80, 80 a and regulationdevices 84, 84 a or a single source and regulation device. Meanwhile thesecond chamber 54 can be pressurized, typically to a greater pressure,to urge the first and third skin members 42, 42 a against the respectivecontour surfaces 15, 17 of the dies 14, 16 and urge the skin members 42,44, 42 a, 44 a against the respective core members 46, 46 a to formbonds between the skin and core members.

The preform 40 can then be removed from the die cavity 18, and twostructural assemblies 70 can be formed from the preform 40. That is, afirst structural assembly, similar to the one illustrated in FIG. 4, canbe formed from portion 70 a of the preform 40 including the first andsecond skin members 42, 44 and the core member 46. Similarly, a secondstructural assembly, which can also be similar to the one illustrated inFIG. 4, can be trimmed from portion 70 b of the preform 40 that includesthe third and fourth skin members 42 a, 44 a and the core member 46 a.In other embodiments, any number of structural assemblies can be formedfrom a single preform. In some cases, the combined manufacture ofmultiple structural assemblies can reduce the amount of scrap material,i.e., material that is trimmed from the preform and does not constitutepart of the finished structural assembly. That is, the preform 40 ofFIG. 6 can be used to form two structural assemblies 70 without trimmingfrom the preform 40 a flat closeout sheet, such as the one illustratedin FIGS. 2 and 2A, which is not part of the finished structural assembly70.

Many modifications and other embodiments of the invention will come tomind to one skilled in the art to which this invention pertains havingthe benefit of the teachings presented in the foregoing descriptions andthe associated drawings. Therefore, it is to be understood that theinvention is not to be limited to the specific embodiments disclosed andthat modifications and other embodiments are intended to be includedwithin the scope of the appended claims. Although specific terms areemployed, they are used in a generic and descriptive sense only and notfor purposes of limitation.

1. A method for forming a lip for a nacelle defining a complex contourhaving opposite inner and outer mold line surfaces and defining a centerextending along an arc, the method comprising: forming a first skinmember to substantially define a desired convex contour of the outermold line surface of the lip; forming a second skin member tosubstantially define a desired concave contour of the inner mold linesurface of the lip; subsequent to said forming steps, disposing acellular core member between the first and second skin members andconnecting a periphery of the skin members to form a preform defining afirst gas chamber between the skin members; disposing the first skinmember against a contour surface of a die; providing a pressurized fluidagainst the second skin member to urge the second skin member toward thecontour surface of the die, thereby urging the first skin member againstthe contour surface of the die; injecting a pressurized fluid in thefirst chamber to support the cellular core member; and heating thepreform to a bonding temperature such that the skin members and the coremember are bonded to form the lip with the preform restrained againstthe contour surface of the die.
 2. A method according to claim 1 whereinsaid forming steps comprise superplastically forming the first andsecond skin members to substantially define the desired contour of thelip of the nacelle.
 3. A method according to claim 1, further comprisingforming the cellular core member to correspond to the shape of the skinmembers and the desired contour of the lip of the nacelle.
 4. A methodaccording to claim 1, further comprising providing the first and secondskin members and the core member, each of the first and second skinmembers and the core member comprising titanium.
 5. A method accordingto claim 1, further comprising providing at least one of the skinmembers of a material that is dissimilar to the material of the coremember.
 6. A method according to claim 1 wherein said step of providinga pressurized fluid against the second skin member comprises providing afluid pressurized to at least about 10 psi.
 7. A method according toclaim 1 wherein said injecting step comprises pressurizing the firstchamber to a pressure of between about 0 and 1 psi less than thepressure provided against the second skin member.
 8. A method accordingto claim 1, further comprising connecting a closeout member to thesecond skin member opposite the first skin member such that the closeoutmember and the second skin member define a second chamber, and whereinsaid step of providing a pressurized fluid against the second skinmember comprises injecting a pressurized fluid in the second chamber. 9.A method according to claim 8, further comprising repeating said formingsteps and said disposing steps to form the closeout member having acellular core member between third and fourth skin members, and whereinsaid connecting step comprises forming the second chamber between thesecond skin and the closeout member such that the cellular core memberof the closeout member is bonded to the third and fourth skin members ofthe closeout member.
 10. A method according to claim 9, furthercomprising providing at least first and second dies and closing the diesto form a die cavity, such that the first skin member is restrainedagainst the contour surface defined by the first die and the closeoutmember is restrained against a contour surface defined by the seconddie.
 11. A method according to claim 1 wherein said second disposingstep comprises disposing the preform in a die assembly having at leasttwo dies configured to be closed to form a substantially sealed space.12. A method according to claim 1 wherein said heating step comprisesheating the preform to a diffusion bonding temperature of the preformand thereby diffusion bonding the core member to the skin members.
 13. Amethod according to claim 1, further comprising providing a brazematerial between the core member and the skin members, and wherein saidheating step comprises heating the preform to a brazing temperature ofthe braze material and thereby brazing the core member to the skinmembers.
 14. A method for forming a structural assembly defining acomplex contour, the method comprising: forming a first skin member tosubstantially define a desired contour of a first side of the structuralassembly; forming a second skin member to substantially define a desiredcontour of a second side of the structural assembly; subsequent to saidforming steps, disposing a cellular core member between the first andsecond skin members and connecting a periphery of the skin members toform a preform defining a first gas chamber between the skin members;disposing the first skin member against a contour surface of a die;providing a pressurized fluid against the second skin member to urge thesecond skin member toward the contour surface of the die, thereby urgingthe first skin member against the contour surface of the die; injectinga pressurized fluid in the first chamber to support the cellular coremember; and heating the preform to a bonding temperature such that theskin members and the core member are bonded to form the structuralassembly with the preform restrained against the contour surface of thedie.
 15. A method according to claim 14 wherein said forming stepscomprise superplastically forming the first and second skin members tosubstantially define the desired contour of the structural assembly. 16.A method according to claim 14 wherein said step of providing apressurized fluid against the second skin member comprises providing afluid pressurized to at least about 10 psi.
 17. A method according toclaim 14 wherein said injecting step comprises pressurizing the firstchamber to a pressure of between about 0 and 1 psi less than thepressure provided against the second skin member.
 18. A method accordingto claim 14, further comprising connecting a closeout member to thesecond skin member opposite the first skin member such that the closeoutmember and the second skin member define a second chamber, and whereinsaid step of providing a pressurized fluid against the second skinmember comprises injecting a pressurized fluid in the second chamber.19. A method according to claim 18, further comprising repeating saidforming steps and said disposing steps to form the closeout memberhaving a cellular core member between third and fourth skin members andwherein said connecting step comprises forming the second chamberbetween the second skin and the closeout member such that the cellularcore member of the closeout member is bonded to the third and fourthskin members of the closeout member.
 20. A method according to claim 19,further comprising providing at least first and second dies and closingthe dies to form a die cavity, such that the first skin member isrestrained against the contour surface defined by the first die and thecloseout member is restrained against a contour surface defined by thesecond die.
 21. A preform for forming a structural assembly defining acomplex contour, the preform comprising: a first skin member defining acontour of a first side of the structural assembly; a second skin memberdefining a contour of a second side of the structural assembly, thesecond skin member connected to the first skin member to define a firstgas chamber between the first and second skin members; a core memberdisposed in the first gas chamber and defining a plurality of cellsbetween the first and second skin members; a gas connection in fluidcommunication with the first chamber and configured to deliver apressurized fluid to the first chamber for supporting the core member inthe first chamber; a closeout member connected to the second skin memberopposite the first skin member and defining a second gas chamber betweenthe closeout member and the second skin member; and a gas connection influid communication with the second chamber and configured to deliver apressurized fluid to the second chamber to urge the preform against acontour surface of a die.
 22. A preform according to claim 21 whereinthe core member is bonded to each of the skin members by at least one ofdiffusion bonds and braze joints.
 23. A preform according to claim 22wherein at least one of the skin members is formed of a material that isdissimilar to the material of the core member.
 24. A preform accordingto claim 21, wherein the closeout member comprises third and fourthcontour skin members connected to define a third gas chamber between thethird and fourth contour skin members and a core member disposed in thethird gas chamber and defining a plurality of cells between the thirdand fourth skin members, the third and fourth skin members being joinedto the first and second skin members to form the second gas chamberbetween the first and third skin members.